How Galaxies Are Disappearing From Our Universe 174
StartsWithABang writes: You've heard of dark energy before, and you know that it causes the expansion of our Universe to be accelerating. Instead of slowing down, distant galaxies are speeding up in their recession from us, rendering them unreachable from our point of view. But even though we can't see the light emitted from them today, we can still see the galaxies themselves! This article explains how this works, how no information gets lost, and what it means for the Big Bang.
Balloons (Score:5, Interesting)
What always bothered me about the balloon analogy was the implication that this expansion of space is mostly taking place where there's little actual matter, ie the space between galaxies. If it really was expanding like dots on a balloon, we'd see equivalent expansion within galaxies and as far as I'm aware we don't, at least not to any significant degree.
Actually the whole thing is bothersome, if a galaxy was x light years away at some point in the past and it's now 2x light years away due to space expanding, doesn't that mean space has been created between the galaxies, and doesn't that violate some fairly fundamental laws of physics?
Balloons (Score:5, Interesting)
What always bothered me about the balloon analogy was the implication that this expansion of space is mostly taking place where there's little actual matter, ie the space between galaxies. If it really was expanding like dots on a balloon, we'd see equivalent expansion within galaxies and as far as I'm aware we don't, at least not to any significant degree.
At that scale, gravity massively dwarfs expansion. For any system which is gravitationally bound, you can assume the "force" of expansion is trivial.
Actually the whole thing is bothersome, if a galaxy was x light years away at some point in the past and it's now 2x light years away due to space expanding, doesn't that mean space has been created between the galaxies, and doesn't that violate some fairly fundamental laws of physics?
I think you understand. Yes, the hypothesis is that space itself is being created, and that this is a fundamental law of physics. There's no fundamental law for it to violate, there's conservation mass and energy, no conservation of space.
Re: Balloons (Score:2)
But if space is full of virtual particles which represent energy and mass, then you are saying that energy and mass are created from nothing. No?
Re: Balloons (Score:5, Interesting)
No. At least I don't think so. I'll admit my understanding is fuzzy, but a few points to think on:
As I understand it (poorly), virtual particles appear to violate mass/energy conservation by their very nature - but they don't actually possess mass or energy themselves. Instead they sort of run on an "energy debt" for a few moments before either annihilating or capturing some "normal" energy so that they can persist. I think the energy is believed to be borrowed from the vacuum energy field - so that essentially you've temporarily lowered the energy of space itself in order to create some virtual particles, and there is no net energy change within the volume.
Vacuum energy is *not* conserved - like dark energy it is created along with new space-time. Again this simply appears to be one of the fundamental laws of the universe - mass/energy conservation apparently doesn't apply to space-time itself, only to things *within* space-time.
And finally there's a real possibility that matter can be created from whole cloth without violating conservation: In creating mass you also create a gravitational field, which in turn reduces the gravitational potential energy of everything else in the universe (gravitational energy is always negative). That reduction in the potential energy of the entire universe *may* perfectly balance the increase in mass-energy of the new matter. This is actually one of the theories about the big bang - it may be that the entire universe, taken as a whole, contains exactly zero net mass-energy: in essence everything really was created from nothing, and if you squeezed it all back together again you'd find there was still nothing there. It's just that the nothingness has been separated from itself in such a manner that things can be built out of the pieces.
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Much like the very small where we can still only make guesses, the very big also is something we can only guess about. What is really happening between galaxies, the place where all the solar winds from billions of suns within each galaxies goes to die and what impact does it have on light as it travels those distances. Likely things are happening between galaxies that we for quite some time will only ever be able to guess at. It is not like they are empty, after all the suns from each galaxy are pouring h
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Well, it's pretty close to completely empty, estimates for the mass-energy density of the intergalactic medium put it at roughly equivalent to one proton per cubic meter - compared to the one proton per cubic centimeter of interstellar space, or the . And of course the vast bulk of the universe's volume is actually inter-cluster space (or probably more accurately and inter-super-cluster space), and you'll probably get a similar density drop off as you move into each progressively more isolated region.
For
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Oh, though I suppose it's also worth mentioning the possibility that our universe exists as a false-vacuum, in which case there may be immense energies existing in the fabric of space-time just waiting for a single point to decay into a lower-energy state, releasing (some?) of those energies as new matter and energy, and catalyze a vacuuum decay cascade that would sweep through the universe at almost the speed of light, destroying everything in it's path in a new "big bang". It's theorized that that may ha
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Perhaps those mysterious and very distant massive gamma ray bursts without the China syndrome chaser might be related?
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Probably not - if the cascade propagates at just under the speed of light, then our first warning would probably be an insanely rapidly expanding disc of "fire" consuming the night sky as the nearest edge of the interface between universes rushed towards us, creating a seething expanse of mass-energy from the void itself - assuming photons are able to cross the boundary at least. Otherwise we'd probably see a blotting out of the stars instead, possibly accompanied by scintillating reflections of stars stil
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AC wrote
But there is conservation of angular momentum. If you insert extra space between two objects that are rotating about a common center of mass, then either their rotation must slow down, or you are getting fee energy from the expansion of space. I'm not personally
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How would you go about observing this? For that matter, how could you tell how fast the universe was spinning (and in which dimensions)?
Perhaps the universe is spinning in n dimensions around some axis in the n+1th dimension. That would seem to mean that there would be a force seeking to expand the universe acting perpendicular to the axis in every dimension less than n. Unfortunately, it would also seem to mean that the force acting along different dimensions could well be uncorrelated in magnitude, so
Re:Balloons (Score:5, Interesting)
As the AC's have said, gravity and other forces between particles completely swamp the "force" of expansion - for now. Imagine putting sheets of paper on a rubber sheet, and stretching the sheet. To an approximation, the centers of the pieces of paper would remain on the same spot on the sheet, while the rest of the paper would have the rubber sheet sliding out from under it, but the piece of paper would resist this "pull" on its edges easily.
If the expansion of space is expanding, though, there will come at a time when this "force" (if it is a force, I'm the wrong person to ask) becomes more significant. First, gravity won't be able to hold galactic clusters together, then galaxies, then solar systems, then the forces between and within atoms won't be enough to keep them together.
Sorry, bit of a downer to end on.
'Big Rip' better than Heat Death (Score:2)
Sorry, bit of a downer to end on.
Not really. Before we had Dark Energy the ultimate fate of the universe was to expand up to a finite size and sit there for ever until all the stars died and the Black Holes evaporated leaving and empty, dead universe going on forever.
Now we have an unknown fate since we have no idea what will happen when the Dark Energy density causally disconnects points at the Planck-length, the so-called "Big Rip". I'll take the unknown over permanent, eternal heat death any day.
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Well, either that, or the "Big Crunch", or infinite expansion at an ever-decelerating rate. The "Critical universe" you describe was always vaguely improbable - one atom more or less and you end up with one of the other two. You almost need a deity fine-tuning things for it to be a viable option. We discovered dark energy accidentally when trying to determine which of the three possible outcomes mandated by existing physics was most likely.
It had been assumed that the universe was in a steady state primar
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I don't buy the religious input at all though. The reason for assuming a steady state universe was simply because the local universe appears relatively constant and unchanging i.e. in a steady state. It is only when you look at the largest possible scales that you realize that things
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Being an atheist in no way insulates you from religious-based preconceptions - at least not when those preconceptions are interwoven into fundamental, unstated assumptions upon which the culture you grew up in is based.
Take the idea of a clockwork universe as another example - which pervaded western thought for many centuries, and still does among many armchair physicists and a few professional grade theoreticians. Such a concept is directly in line with the idea that the universe was made perfect by God,
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Of course, the big rip would give us a reason not to procrastinate for billions of years, so I guess there's something to be said for h
Re:Balloons (Score:5, Interesting)
Not quite.
The rate of dark-energy expansion is not actually changing - it's simply that as things drift apart you get more and more space between them, and since the space itself is expanding, the further apart you are, the more total expansion is occurring between you. Like stretching a rubber band with a couple marks near the middle - the ends will be getting further apart at a much faster rate than the marks near the middle, even though the entire rubber band is stretching at a constant rate.
That means anything that is *already* drifting apart will accelerate apart indefinitely, but if the forces between two objects are currently holding them at a fixed distance from each other, then the amount of space (and dark energy) between them is not changing, and thus the repulsive "force" of expansion between them will likewise remain unchanged.
Now for galaxies and even star systems - they're constantly shedding mass and energy, so *eventually* the gravitational bonds that hold them together may weaken to the point that they fly apart. And once that happens, and the space between objects begins to increase, only then can expansion begin to accelerate them apart even faster. For individual planets and stellar-remnants though, gravity is more than sufficient to keep the individual pieces in place.
As for atoms... the distances between subatomic particles are fundamental - you can't raise an electron further from the nucleus without adding an extremely precise amount of energy as a photon, and it will rapidly shed that photon to return to its stable base state. It's simply not possible for things to "drift apart" - they can only exist at very precise distances determined by their quantum wavefunctions. And since things can't get drift apart, expansion can't get it's foot in the door. Atoms have nothing to fear from expansion except loneliness.
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The rate of dark-energy expansion is not actually changing
Isn't it? I thought we still didn't know.
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True, I spoke from a point of ignorance. From what I can tell though it seems like most of the people arguing for a changing rate of expansion are arguing that it's slowing down rather than speeding up. In any case though, I've yet to find a reputable source on the subject.
Re:Balloons speckled with paint... (Score:5, Interesting)
A spot of paint on your balloon would locally restrict expansion as it inflates, as galaxies seem to do in our expanding universe. My understanding of current hypotheses is that dark matter plays the role of "paint" in this analogy. However, there's an intriguing alternative explanation, which only becomes apparent when you think of space as a fluid. [scientificamerican.com]
Ironically, I stumbled upon this notion after musing on the strong interaction. [wikipedia.org] (And I confess I was a bit high at the time.) Something that repels at a distance but attracts in proximity... that reminds me of bubbles interacting in the surface tension of fluids. So I googled "space as a fluid" and found that there's a whole branch of inquiry in this direction. It doesn't get as much attention as String Theory, but it's not dismissed out of hand either. (Correct me if I'm wrong... IANA physicist.)
Anyway, I'm curious to hear others' thoughts on this.
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What I've wondered, and I don't expect to get a satisfying response here, but could the balloon analogy be used to explain why space is expanding between galaxies. Like imagine you had a balloon that wasn't expanding, but was just filled up with a fixed amount of air. Now everywhere that you've drawn a galaxy, you're start pinching the balloon. Like somehow you find a way to squeeze and compress those places where a galaxy is drawn, and you don't let it rebound. You just keep pulling and pinching and s
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Gravity slows down the expansion, and, it slows down the passage of time. Time and space are not merely relative they are one and the same. The expansion of space and the passage of time -are the same thing- .
Perhaps at some point space-time expands so fast that it actually rips and a huge expanse of the universe is flooded with quantum energy made matter. Uniformly, throughout the expanse. This matter then slows down the expansion -temporarily though, as matter collects together due to gravity and crea
Re:Balloons (Score:5, Interesting)
"doesn't that violate some fairly fundamental laws of physics?"
Do you not think that one of the many thousands of theoretical and observational physicists who've worked on this model for decades would perhaps have spotted this flaw at some point in the last eighty years...? Of course it doesn't violate fundamental laws of physics. The whole thing is based tightly on general relativity, so regardless of whether you believe that relativity is being applied accurately to cosmology or not (I don't, not entirely) there is no suggestion of it violating any fairly fundamental laws. Conservation of mass/energy is absolutely guaranteed in relativity. (In two tightly-coupled ways - directly, and via the Bianchi identities which are nothing more than geometric identities along the lines of, but more complicated than, the Pythagoras theorem. Which one you take as more fundamental depends on your philosophy but in relativity the one implies the other.)
The balloon analogy is basically flawed. It's also flawed because it relies on one imagining (to the extent that one can, and no-one can actually do so since our brains didn't evolve to imagine 4d let alone 5d) a 3+1d balloon embedded in a 4+1d spacetime, through the analogy with a 2d balloon embedded in 3d space. This inevitably leads to people understandably querying where the centre is and wondering if it's in the middle of this 4+1d space. It also leads people to understandably ask why the galaxies aren't expanding.
Basically, they're not expanding because the theory doesn't apply in them. There are two ways of viewing this - the simple (but inaccurate) and the headfuck. The simple way of looking at it is that the cosmological expansion is extremely weak and is very easily overpowered by other, more local, forces. So galaxies are easily held together because the gravitational pull between stars in a galaxy is overwhelmingly stronger than the pull of the cosmological expansion. This, unfortunately, does suggest there's some kind of balancing of forces and some kind of spatial expansion, which isn't strictly speaking true.
The headfuck is something that's actually almost impossible to model but straightforward to understand in relativity. The theory that the balloon analogy is based on is Friedman-Lemaitre-Robertson-Walker (FLRW - we're probably missing a name or two in there, as well) cosmology, based on what's known as the FLRW metric, which does nothing more than give the Pythagoras theorem in a 3+1d universe made up of an inverted pyramid of flat 3d spatial surfaces stacked one on top of the other along some time direction. (They could also be a load of nested spheres, or more bewilderingly a pile of saddles, but the data supports the flat model and there's currently no real reason to favour the so called closed or open models.) The FLRW metric applies on scales at which the universe seems to look the same in every direction and wherever you move to. In the jargon, it's "homoegenous and isotropic". Things like the SDSS surveys demonstrate how this can happen quite well -- take a look at http://www.a.phys.nagoya-u.ac.... [nagoya-u.ac.jp] which is the collection of data from the first SDSS survey (which ended about a decade back, I think; we're on SDSSIII or thereabouts now but I like this figure). On small scales this is obviously really knotty and far from homogeneous, but if you zoom out and squint slightly (to give a form of smoothing) then everything looks the same. Doing this a bit more rigorously, which is notoriously model-dependent, gives the "homogeneity scale" at somewhere in the order of 100Mpc, or about a hundred times larger than a typical galaxy cluster. That's the scale at which the FLRW model applies -- and that's the scale at which every single consequence can be said to hold. Below that, nothing that it says should be taken without a massive pinch of salt. This is particularly true in clusters, which are what is known as 'virialised' and detached from the cosmological expansion -- t
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A bit of both an objection to FLRW itself, and to dust solutions. On a fundamental level, the FLRW solution can never be more than an approximation to the real universe until we find a way of mapping the small scale physics up to a universal scale - since no well-defined averaging procedure exists (regardless of whether we're doing 3d averaging, 4d averaging, or some kind of statistical averaging) that does this with any rigour and generality, we can't actually ever state that the universe is FLRW "on avera
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It's impossible to say for certain at the minute since we can't prove anything, but my guess would be both -- we don't understand what can generate a metric on cosmological scales (in the sense of how it's composed of billions of billions of metrics that are best modelled by Schwarschild or Kerr-Newman), but if we did understand how to do so it would most likely by necessity still be a phenomenological description. The ideal would be that we'd end up with a situation similar to that of thermodynamics, which
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if a galaxy was x light years away at some point in the past and it's now 2x light years away due to space expanding, doesn't that mean space has been created between the galaxies, and doesn't that violate some fairly fundamental laws of physics?
I read a book that mentioned our on expansion, this was before dark energy was attributed to it.
It stated that E=MC^2 also means Energy can be converted into Mass, and in the universe particles are popping up all over the place, and mass is gravity (not that this mass couldn't be dark matter).
The cite -I'd use besides the books name (I've read so many on Quantum Physics I'm at a loss to name to name it) - or hidden within this link http://en.wikipedia.org/wiki/M... [wikipedia.org]–energy_equivalence
I notice a submiss
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It stated that E=MC^2 also means Energy can be converted into Mass, and in the universe particles are popping up all over the place, and mass is gravity (not that this mass couldn't be dark matter)
Energy also can cause gravity, so the conversion process involved in e=mc^2, e.g. photons converting to a positron-electron pair, would not change total gravity involved (short of the fact you've now spread things out to different places).
No it might not change the total gravity involved but it would localize it.
It takes something to interact for this to happen, the interacting forces being mobile prior to this.
I'm not short on fact, just not as good a memory as I once had; it's replies like yours that help. and for that I thank you.
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There is no conservation of spacetime, so no violation of anything.
The space within a galaxy expands as well, but it doesn't amount to much Compared to intergalactic space, there's not much space in a galaxy. The expansion is counterbalanced by gravity.
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the expansion rate is believed to be about 75 kilometers per second per 3 megaparsecs (about three million light years). That is very small
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> What always bothered me about the balloon analogy
That's because it's an analogy - they're not meant to be perfect representations of reality.
Re: Balloons (Score:2)
At the same point, whose to say that over the time scales we're talking about that galaxies, solar systems and everything *haven't* increased in size proportionally relative to the expa
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He thinks it violates conservation of mass/energy. It doesn't, of course, but it's not entirely unreasonable to ask whether it does if you don't know where the theory actually comes from.
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If space is expanding everywhere, including the distance between atomic sub particles, wouldn't they collapse again due to gravity/electromagnetic forces back to the normal energy well positions?
That would generate energy out of nowhere, making every atom, nucleus and baryon unstable.
Re: Balloons (Score:2)
Save the Galaxies! (Score:5, Funny)
I am personally doing my part to conserve galaxies and I hope that all of you are too. Please, please, please help do your part to conserve this valuable resource before it is too late. Not just for today because it's Universe Day, but for life.
Where's Sarah MacLachlan when you need her? (Score:2)
Re:Save the Galaxies! (Score:5, Funny)
100 000 000 000 should be enough galaxies for anyone.
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I just donated 25 Dogecoin to the Save the friggin' Galaxies project to show how much I care!
What? (Score:1)
Re:What? (Score:5, Informative)
Or not (Score:2)
You've heard of dark energy before, and you know that it causes the expansion of our Universe to be accelerating.
How do you know? Maybe I haven't, and maybe I don't.
This article explains [...] how no information gets lost
I'll admit I only skimmed the article - it is medium.com after all, and contains a ridiculous seven exclamation marks (plus an interrobang and a double exclamation mark in an image note, for pity's sake) - but I didn't see where this got explained.
Curious (Score:1)
I read quite often that galaxies are moving away from each other at increasing speed.
In fact faster than light.
While special relativity constrains objects in the universe from moving faster than light with respect to each other when they are in a local, dynamical relationship, it places no theoretical constraint on the relative motion between two objects that are globally separated and out of causal contact. It is thus possible for two objects to become separated in space by more than the distance light could have travelled, which means that, if the expansion remains constant, the two objects will never come into causal contact. For example, galaxies that are more than approximately 4.5 gigaparsecs away from us are expanding away from us faster than light. We can still see such objects because the universe in the past was expanding more slowly than it is today, so the ancient light being received from these objects is still able to reach us, though if the expansion continues unabated, there will never come a time that we will see the light from such objects being produced ‘'today (on a so-called "space-like slice of spacetime") and vice-versa because space itself is expanding between Earth and the source faster than any light can be exchanged.
So that's confusing to me, wouldn't their mass increase as well and possibly lead to a massive attraction then collapse of the Universe back to the point prior to the Big Bang?
Or is it just the distance not the velocity relative to each other.
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I going to take shot at this and press my luck, as I've already posted a comment about cosmology which is beyond my real understanding -- but ... The apparent mass increase you refer to is a Special Relativistic effect and Special Relativity only holds strictly in a flat, non-expanding space-time. Once you bring in General Relativity and curved or expanding space-times then your notions from Special Relativity don't hold, especially at cosmological distances. And the relation of velocity to red shift is
How galaxies are disappearing from our universe. (Score:2, Insightful)
Granted we can no longer see them, but that's a pretty arbitrary assumption to say that they have disappeared from our universe, any more than it's okay to say that it's us who disappeared. Even if we can't see them, it's a safe bet that they're still governed by the same laws of physics we are. It would be really strange if their (or our) laws of physics suddenly changed just because we can't see each other any more. Not being able to see each other is just one consequence of those laws in the universe w
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Well, it's a lot more than simply disappearing from *view* - passing behind a dense dust cloud would do that. They are disappearing from all causal contact with us - unless special relativity is wrong, and it *is* possible to travel faster than light (with all the causality-breaking problems that would entail) those galaxies no longer exist from our perspective: it is theoretically impossible for there to be any further interaction between us, ever. For all intents and purposes they have completely ceased
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Can there still be interaction between the middling galaxy and us? Yes.
Just because we can't interact directly with it doesn't mean that all influence and interaction is gone.
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Can there still be interaction between the galaxy that just disappeared, and a galaxy mid-way between us? Yes.
Can there still be interaction between the middling galaxy and us? Yes..
Both true, but these interactions don't combine. Suppose you have three galaxies in a line A--- B---C and A and C are just leaving causal contact.
Suppose a light-speed message is sent from A towards B and C. B will indeed receive it, and be able to reply to it (maybe) but that will happen just as B and C leave causal contact (the universe having carried on expanding), so that if that message is forwarded towards C it will still not arrive. The photons in the forwarded message cannot overtake those in the or
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If B and C are close enough to be gravitationally bound then A will lose contact with both of them at the same time.
Objects don't have to be gravitationally bound to influence each other. A rogue plantoid passing through our system isn't gravitationally bound to it, but our gravity still can modify its path.
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If B and C are close enough to be gravitationally bound then A will lose contact with both of them at the same time.
Objects don't have to be gravitationally bound to influence each other. A rogue plantoid passing through our system isn't gravitationally bound to it, but our gravity still can modify its path.
You're right, but you've misunderstood my point. If A, B and C are all "far" apart then all the distances are increasing at an accelerating velocity and the situation is as I described it. The last paragraph deals with the special case where B and C are close enough that they are not accelerating apart. In this case B and C will remain in contact forever, and so A will lose touch with both of them at the same time.
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To get very far away from us they started receding from us at a higher speed than objects that are closer. However, nobody can point to where an object "disappeared" - it's all conjecture unsupported by experiment or direct observation. Who knows, maybe when the fabric of the universe gets too thin, the repulsive force becomes an attractive force. We simply don't know enough yet.
Of course. Anything could happen, but there is a remarkably consistent, and mathematically simple, if somewhat unintuitive picture emerging of how the universe has evolved on the largest scales. The picture in general (dark matter, dark energy, etc,) is consistent with a number of independent sets of data, for example supernova surveys and detailed analysis of the cosmic microwave background. The article is trying to explain the consequences of this picture.
What we can see are galaxies at very high redshif
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Could do, although there is no evidence of such an effect up to now. The laws of physics could also just change tomorrow for no particular reason, in thi sarea, or in some much more down-to-earth one, like whether the proton is stable. We can never know.
The article is essentially in the business of explaining the consequences of the laws as we currently conjecture them to be (which fit what we can observe pretty well). It can't make any stronger claim to be "correct" than that, but, apart from refining "pre
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But they weren't interacting with each other's *present*. When we interact with a galaxy a billion light years away, we're experiencing the influence of what they did a billion-plus years ago, while they experience the influence of what we did a billion-plus years ago.
They may still be under the influence of "Galaxy 3" that has already left our Hubble Sphere, but it will be the influence of that galaxy's distant past, while it was still within our Hubble Sphere. By the time they get influenced by Galaxy 3'
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Yep. From the article:
"And while no galaxy has literally disappeared to the point where it's invisible, 97% of them have disappeared in the sense that they're unreachable to us, and that the light they're emitting today will never reach us. The galaxies are still visible, but only due to their old light."
They're not disappearing from THE universe, they're disappearing from OUR universe.
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And they interact with others that are closer to us.
Wash, lather, rinse, repeat enough times, and they still indirectly interact. They have not exceeded C in relation to all their neighbors.
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Doesn't work. If you try and relay light (or any other message) along the line from the distant galaxy to us, what happens is that it reaches each relay station just as the relay station loses contact with us. It never arrives.
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What sort of proof do you want, and how does gravity come into it? Happy to try.
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Obviously at one point these objects were closer together. Therefore, they had some (maybe infinitesimal, but still not zero) effect on each other, either through bending of space/time, or through gravitons (take your pick which). If two galaxies are receding from each other in exactly opposite directions, and from a frame of reference between the two each is apparently receding at 2/3 c, then neither object appears in the other's frame of reference. However both can influence, and be influenced by, the obj
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If you're dealing with constant velocities, you are in the territory of special relativity. In this world there are no event horizons and every object can interact with every other. If two galaxies are each receding in opposite directions from a third central one at 2/3 c they will each see the other receding at 12/13 c according to https://en.wikipedia.org/wiki/... [wikipedia.org] (section 2). Velocities do not add up the way you think they do and when they get to a decent proportion of light speed it starts to matter. Th
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I noticed that you ignored the case of objects approaching each other, each with a substantial fraction of the velocity of c. I used that thought experiment to pose the question of how they interact, because we know from observation that they do, even though each one is approaching the other faster than c. From the frame of reference of one, the other never exits, unless time and space are not continuous at the smallest scales.
Let's make it two planets approaching each other, as seen from an observer sligh
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Constant approaching velocity is special relativity again, and again the velocities don't add the way you expect.If the planets in your example are approaching at 2/3 c they each see the other approaching at 12/13 c and they will very definitely and messily interact. Each exists for the other.
In this case acceleration makes no essential difference though. In either planets frame of reference there is an event horizon behind it (in GR acceleration and gravity are equivalent) but none in front of it, so they
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And why would you ignore the semantics of my sentence except to be obtuse? Quite likely my meaning was understood by everyone but you.
You and your hair-splitting ilk are what take the fun out of online interaction.
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disappear [verb]
1) to cease to be seen; vanish from sight.
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That doesn't have to do with things changing because you could see something or not, but because you keep forcing it into a specific state through interaction.
Seeing IS interacting :-)
The long and short of it. (Score:5, Interesting)
The universe is expanding.
As it expands, attractant forces (like gravity) hold less and less sway over things.
Without that "drag", more distant objects are speeding up.
We're starting to get to the point that certain objects are far enough away that, unless we find a BIG loophole in physics someplace, we'll never be able to reach them. And unless we find it SOON, we'll lose track of these objects, thus pretty much negating our ability to plot a course to them at all.
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Actually we're not "starting to" get to that point, I believe the article stated that 97% of observable galaxies have *already* crossed that threshold - we're just still seeing the light that was emitted (mostly) long before they did so. And, like watching something fall into a black hole, we will continue to see their fading, increasingly red-shifted image for as long as we can construct instruments sensitive enough to detect the photons emitted in the last instants before it crossed the "event horizon".
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No the increase of the expansion velocity is mainly because of the creation of space in between galaxies, not because of a decreased effect of gravity.
This is one of the weird properties of what is being observed, because it implies that our universe will be increasingly dominated by dark energy. Current dark energy accounts for 70% of the energy balance of the universe. With the space being created also containing 'new' dark energy this number will increase in the future, quite possibly exponentially based
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The speed of light limit applies to matter flying in space, it doesn't apply to the expansion of space itself. The inflationary epoch shortly before the Big Bang is an example of this
What happens to the photons? (Score:2)
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I'll probably get busted by some cosmologist for this, but -- there is no "edge" to the Universe, at least not one a photon or anything else can can travel to. For one thing, if you try to look far enough in any direction, from anywhere, you will eventually see space expanding away from you faster than light so the photons at the speed of light can go forever without catching up. More philosophically, because the Universe is "practically" (this is where I will get busted) defined by the space-time manifol
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As a cosmologist, I can comment that the entire theory being talked about is based on a particular solution of GR, which involves defining a particular metric living on a manifold and endowing it with dynamics via the Einstein field equations - meaning I don't have much argument with what you said.
Appearences can be decieving. (Score:2)
What we see is that the expansion of the universe appears to be accelerating, but what if there's another explanation?
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but what if there's another explanation
Then we'll consider it.
First it was planets ... now galaxies (Score:2)
They're probably just being reclassified as "dwarf star conglomerations" or somesuch.
QM (Score:2)
Perhaps somebody can explain this to me.
If in Quantum Mechanics anything can happen with a certain (perhaps very small) probability, then in an infinite amount of time anything will in fact happen.
This proves that the universe will in fact never collapse.
Or does it?
What tfa fails to mention (Score:2)
Galaxies Moving Away? Really? (Score:2)
Alright all you experts out there riddle me this one. If the universe is expanding then exactly why are the Milky Way and Andromeda Galaxies supposed to collide in the future????
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This should be modded funny not interesting, seriously people (who have mod points)
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Nope, the heavy accelerations away from ours started the day microsoft windows was first announced.
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It's more likely they can't stand the insufferable whiners on the internet who can't stop talking about how horrible the world is.
The ping time would be horrendous,
The energy required tremendous,
But they don't want your stash of p0rn
To them naked walrus are stupendous.
Burma Shave
What would happen if the expansion of our knowledge outpaced the expansion of the universe? Is there a cross-over point, so that we (or our robotic descendants) will be able to literally control the universe? And if so, should we?
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Or perhaps GP is a really sly Samsung slashvertisment.
"World so bad, I just need to feel my Galaxy S5!"
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That sounds like an intro to a "Look Around You" style pseudo-science humor piece. If so, please point me to it, I could go for a good laugh.
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Ah. I've got to get back into watching those - I'm a long-time fan but it's just not worth paying for cable/Netflix/etc for a single show.
The Hubble non-Constant? (Score:2)
I'm no expert, but I'm pretty sure he's wrong towards the end when describing the Hubble Sphere "catching up with" light emitted from objects already outside it.
Wikipedia's article on the Hubble Volume states:
However, the Hubble parameter is not constant in various cosmological models[4] so that the Hubble limit does not, in general, coincide with a cosmological event horizon. For example in a decelerating Friedmann universe the Hubble sphere expands faster than the Universe and its boundary overtakes light emitted by receding galaxies so that light emitted at earlier times by objects outside the Hubble sphere still may eventually arrive inside the sphere and be seen by us.[4] Conversely, in an accelerating universe, the Hubble sphere expands more slowly than the Universe, and bodies move out of the Hubble sphere.[1]
Observations indicate that the universe is accelerating,[6] so that some objects that we can currently exchange signals with will one day cross our Hubble limit.
So it sounds like he's describing events that could happen in a decelerating universe, but evidence suggests that we're not in such a universe, so it's irrelevant. As I understand it the reason we can see beyond the Hubble limit is that, when the light we're seeing was emitted, it was actually still w
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As he says (Veritasium) in a comment to his video.
"Some people have been sending wikipedia references saying parts of this video are wrong, but I think it's wikipedia with the misconceptions. For further reference check out this paper: http://journals.cambridge.org/... [cambridge.org]"
I don't know where this misunderstanding has come from.
Re:Where is the center? (Score:5, Interesting)
Yes you are, and because you're not educated in the field.
"The article assumes that planet earth is the center of the universe"
No it doesn't. Cosmology does not assume that the Earth is in the centre of the universe. It assumes the exact opposite. It's even known as the "cosmological principle" -- and it's a fundamental axiom in cosmology. Without it we wouldn't have the model that we're talking about. Instead we'd have Lemaitre-Tolman-Bondi models, which are isotropic around the Earth but definitely not homogeneous.
Basically, building the cosmological model goes like this:
1) Observe the CMB. This is all around us, at 2.7K, and is absolutely the same in every direction. It is, in the jargon, isotropic around the Earth.
2) Assume that gravity on large scales is accurately modelled by a geometric theory of gravity (such as, but not restricted to, general relativity). We now know that on average the universe should be described by a metric that is at least isotropic about a point near to Earth.
3) Since this is obviously absurd, as you've picked up on, apply the cosmological principle. If the Earth is not in a special position in the universe, which it would be an astonishing act of hubris to assume it is, but the universe looks isotropic around the Earth, then there are only two choices. We can either dump the cosmological principle and assume the universe is centred on Earth -- which is... untenable, given the vast scale of the observations -- or we can assume that the universe looks isotropic around every point. This implies that it is homogeneous and isotropic: every point is the same in every way.
4) We can now tighten our previous assumption and assume that the universe is modelled by a metric that is isotropic around every point. That means that it is composed of what are known in the jargon as "maximally-symmetric" 3d surfaces. This leads us naturally and inevitably to the Friedman-Lemaitre-Robertson-Walker metrics, which give rise to the "big bang" theory you dislike so strongly.
There are obviously problems here. The phrase "on average" is used frequently and without rigour. That rigour cannot, as yet, be provided. We have assumed twice the nature of gravity - first that it is geometric in origin, and second that it is described by general relativity, which is basically the simplest geometric theory of gravity. Fitting to observation also leads us, naturally and inevitably -- unpleasantly so, if we're being honest -- to dark energy and dark matter. But there is a need to "create these terms", in that the theory demands them, and the theory is *astonishingly successful*. One of the main successes of FLRW cosmology is that it first predicted a characteristic wavelength of ripples on the cosmic microwave background, which was then observed (and which can be used to determine how much dark matter there is relative to normal matter), and that that same wavelength should also be imprinted on the large-scale distribution of galaxies. This was *also* observed, and is exactly where it was predicted by combining CMB and supernovae observations. This is amazing not least because the theory predicts the CMB forming when the universe is around 300,000 years old, while the large-scale distribution of galaxies is observed when the universe is pushing on a bit, probably around 10bn-12bn years old. The wavelength on the galaxy distribution is therefore extremely stretched compared to that seen on the CMB. And, as one might expect, the level to which it is stretched is extraordinarily sensitive to the cosmology - it doesn't take much of a change in the levels of matter, dark matter and dark energy to put it slap bang in the wrong place entirely.
Doing this unfortunately means we need to put dark energy in the model. Unsurprisingly, this isn't as ad-hoc as it seems, since there are multiple candidates for a dark energy, but it's still a bit unfortunate since not many of them are profoundly appealing. (Perhaps the most appealing is also the original, proposed by Wetterich in 1987, s
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I can't actually imagine a setup that would lead to that in vanilla relativity, but even if we assume it could exist it would introduce strong anisotropies into the universe. The very nature of falling into a black hole introduces a directionality, which would immediately produce anisotropies that we don't observe. If the hole is on a rough order of magnitude in scale with the universe, this would be even more obvious since rather than just having a general directionality constant throughout the universe, w
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Surprisingly, no. There are suggestions of an anisotropy in the Hubble rate in different halves of the sky, but the errors are too big for this to be significant. That's the main problem with doing anything of the sort -- the error bars on the observations are just too big until we get far enough away (as in, taking velocities from galaxies far enough away that there are loads of them) to beat them down by sheer power of numbers.
But what that could let us do is put a constraint on how far away a black hole
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I may not have explained it very clearly. The point is that while near to Earth it's obviously in a "special place" -- no other planet in the entire universe has exactly the same conditions around it, in a relatively sparse arm relatively distant from the centre of a relatively large spiral galaxy in a relatively small galaxy cluster that's on the outer edge of a supercluster -- but that if you zoom out a bit and look at things on average, on scales roughly around a megaparsec in scale and above (which is